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Parsec
The parsec (symbol: pc) is a unit of used in . It is about 3.26 light-years, which is equal to just under 31 (3.1 ) kilometers or just over 19 trillion (1.9 ) miles. The name parsec is "an abbreviated form of 'a distance corresponding to a par'allax of one '''sec'ond'". , Stars, Distribution and drift of, The distribution in space of the stars in Carrington's Circumpolar Catalogue. In: Monthly Notices of the Royal Astronomical Society, Vol. 73, p. 334-342. March, 1913. http://adsabs.harvard.edu/abs/1913MNRAS..73..334D "There is a need for a name for this unit of distance. Mr. Charlier has suggested Siriometer ... Professor Turner suggests PARSEC, which may be taken as an abbreviated form of 'a distance corresponding to a parallax of one second'." It was coined in 1913 at the suggestion of . A parsec is the distance from the to an which has a angle of one arcsecond. History and derivation to an which has a angle of one arcsecond. (1 AU and 1 pc are not to scale (1 pc = 206265 AU)) }} The '''parsec is equal to the length of the side of an imaginary in space. The two dimensions on which this triangle is based are the (which is defined as 1 arcsecond), and the side (which is defined as 1 astronomical unit, which is the distance from the Earth to the Sun). Using these two measurements, along with the rules of trigonometry, the length of the side (the parsec) can be found. One of the oldest methods for astronomers to calculate the distance to a was to record the difference in angle between two measurements of the position of the star in the sky. The first measurement was taken from the Earth on one side of the Sun, and the second was taken half a year later when the Earth was on the opposite side of the Sun. The distance between the two positions of the Earth for the measurements was known to be twice the distance between the Earth and the Sun. The difference in angle between the two measurements was known to be twice the parallax angle, which is formed by lines from the Sun and Earth to the star at the . Then the distance to the star could be calculated using trigonometry. (HEASARC) | work = NASA's Imagine the Universe! | publisher = Astrophysics Science Division (ASD) at 's }} The first successful direct measurements of an object at interstellar distances were undertaken by astronomer in 1838, who used this approach to calculate the distance of .Bessel, FW, "Bestimmung der Entfernung des 61sten Sterns des Schwans" (1838) , vol. 16, pp. 65–96. The parallax of a star is taken to be half of the that a star appears to move relative to the as Earth orbits the Sun. Equivalently, it is the , from that star's perspective, of the of Earth's orbit. The star, the sun and the earth form the corners of an imaginary in space: the right angle is the corner at the sun, and the corner at the star is the parallax angle. The length of the opposite side to the parallax angle is the distance from the to the (defined as 1 (AU)), and the length of the side gives the distance from the sun to the star. Therefore, given a measurement of the parallax angle, along with the rules of , the distance from the sun to the star can be found. A parsec is defined as the length of the side of this in space when the parallax angle is 1 arcsecond. The use of the parsec as a unit of distance follows naturally from Bessel's method, since distance in parsecs can be computed simply as the of the parallax angle in arcseconds (i.e. if the parallax angle is 1 arcsecond, the object is 1 pc distant from the sun; If the parallax angle is 0.5 arcsecond, the object is 2 pc distant; etc.). No s are required in this relationship because the very small angles involved mean that the approximate solution of the can be applied. Though it may have been used before, the term parsec was first mentioned in an astronomical publication in 1913. expressed his concern for the need of a name for that unit of distance. He proposed the name astron, but mentioned that had suggested siriometer and had proposed parsec.Dyson, F. W., "The distribution in space of the stars in Carrington's Circumpolar Catalogue" (1913) , vol. 73, pp. 334–42, p. 342 fn.. It was Turner's proposal that stuck. Calculating the value of a parsec In the diagram above (not to scale), S''' represents the , and '''E the at one point in its orbit. Thus the distance ES is one (AU). The angle SDE is one arcsecond (1/3600 of a degree) so by definition D''' is a point in space at a distance of one parsec from the Sun. By , the distance '''SD is : SD = \frac{\mathrm{ES}}{\tan 1^{\prime\prime}} Using , : SD \approx \frac{\mathrm{ES}}{1^{\prime\prime}} = \frac{1 \, \mbox{AU}}{(\tfrac{2 \pi}{360 \times 60 \times 60})} = \frac{648\,000}{\pi} \, \mbox{AU} \approx 206\,265 \mbox{ AU} . One AU ≈ meters, so 1 parsec ≈ ≈ . A corollary is that 1 parsec is also the distance from which a disc with a diameter of 1 AU must be viewed for it to have an of 1 arcsecond (by placing the observer at D''' and a diameter of the disc on '''ES). Usage and measurement The parallax method is the fundamental calibration step for ; however, the accuracy of ground-based measurements of parallax angle is limited to about 0.01 arcseconds, and thus to stars no more than 100 pc distant.Richard Pogge, Astronomy 162, Ohio State. This is because the Earth’s atmosphere limits the sharpness of a star's image.[http://science.jrank.org/pages/5021/Parallax-Parallax-measurements.html jrank.org, Parallax Measurements] Space-based telescopes are not limited by this effect and can accurately measure distances to objects beyond the limit of ground-based observations. Between 1989 and 1993, the satellite, launched by the (ESA), measured parallaxes for about 100,000 stars with an precision of about 0.97 s, and obtained accurate measurements for stellar distances of stars up to 1,000 pc away. [http://wwwhip.obspm.fr/hipparcos/SandT/hip-SandT.html Catherine Turon, From Hipparchus to Hipparcos] 's was to have been launched in 2004, to measure parallaxes for about 40 million stars with sufficient precision to measure stellar distances of up to 2,000 pc. However, the mission's funding was withdrawn by NASA in January 2002.FAME news, 25 January 2002. ESA's , due to be launched in late 2012, is intended to measure one billion stellar distances to within 20 microarcseconds, producing errors of 10% in measurements as far as the , about 8,000 pc away in the of .GAIA from . Distances in parsecs Distances less than a parsec Distances measured in fractions of a parsec usually involve objects within a single star system. So, for example: *One astronomical unit (AU), the distance from the Sun to the Earth, is . * One gigameter (Gm), same as one million kilometers, is . Like the positive multiple prefixes of parsecs mentioned below, negative multiple prefixes of parsecs can be used, such as microparsecs, which is , and milliparsec, which is . *The most distant , , is 620 μpc away from Earth as of February 2016. It took Voyager 30 years to cover that distance. *The is estimated to be approximately 0.6 pc in . Parsecs and kiloparsecs is thought to be 1.5 kiloparsecs long. (image from Hubble Space Telescope)}} Distances measured in parsecs include distances between nearby s, such as those in the same or . A distance of one thousand parsecs (approximately 3,262 ly) is commonly denoted by the kiloparsec (kpc). Astronomers typically use kiloparsecs to measure distances between parts of a , or within . So, for example: *One parsec is approximately 3.262 light-years. *The nearest known star to the Earth, other than the Sun, , is 1.29 parsecs away. *The distance to the is about 120 parsecs. *The of the is about 8 kpc from the Earth, and the Milky Way is about 30 kpc across. *The ( ) is slightly less than 800 kpc away from the Earth. Megaparsecs and gigaparsecs A distance of one million parsecs (approximately 3,262,000 light-years) is commonly denoted by the megaparsec (Mpc). Astronomers typically measure the distances between neighbouring and s in megaparsecs. Galactic distances are sometimes given in units of Mpc/h (as in "50/h Mpc"). h'' is a parameter in the range 0.5,0.75 reflecting the uncertainty in the value of the ''H for the rate of expansion of the universe: h'' = ''H / (100 km/s/Mpc). The Hubble constant becomes relevant when converting an observed z'' into a distance ''d using the formula d'' ≈ (c'' / H'') × ''z. One gigaparsec (Gpc) is one parsecs—one of the largest distance measures commonly used. One gigaparsec is about 3.262 billion light-years, or roughly one fourteenth of the distance to the of the (dictated by the ). Astronomers typically use gigaparsecs to measure such as the size of, and distance to, the ; the distances between galaxy clusters; and the distance to s. For example: *The is 0.77 Mpc away from the Earth. *The nearest large , the , is about 16.5 MpcMei, S. et al 2007, ApJ, 655, 144 away from the Earth. *The galaxy , observed to have a core similar to the 's, is about 200 Mpc away from the Earth. *The (the boundary of the ) has a radius of about 14 Gpc (46.5 billion light-years). Volume units To determine the number of stars in the Milky Way Galaxy, volumes in cubic kiloparsecs (kpc3) are selected in various directions. All the stars in these volumes are counted and the total number of stars statistically determined. The number of globular clusters, dust clouds and interstellar gas is determined in a similar fashion. To determine the number of galaxies in s, volumes in cubic megaparsecs (Mpc3) are selected. All the galaxies in these volumes are classified and tallied. The total number of galaxies can then be determined statistically. The huge in Astrophysical Journal, Harvard is measured in cubic megaparsecs. In , volumes of cubic gigaparsecs (Gpc3) are selected to determine the distribution of matter in the visible universe and to determine the number of galaxies and quasars. The Sun is alone in its cubic parsec, (pc3) but in globular clusters the stellar density per cubic parsec could be from 100 to 1,000. See also * Conversion of units * * Light-year * * References and notes External links * Category:Units of measure in astronomy Category:Length units Category:Units of distance